Thin film devices may be used to create solar cells, detectors, electronic devices, telecommunication devices, charge-coupled imaging devices (CCDs), computers, and even biological or medical devices (together considered “thin-film compound semiconducting materials”). With regard to renewable energy, solar cells are photovoltaic (PV) devices that have characteristics that enable them to convert the energy of sunlight into electric energy. The aim of research often is to achieve solar cell designs with the lowest cost per watt generated by the solar cell, and, concurrently, the designs should provide solar cells that are suitable for inexpensive commercial production. With regard to this latter concern, it is often difficult to provide adequate quality control for the various layers of the thin film PV device as it is being fabricated or in “real time.”
For example, when manufacturing a semiconductor or thin film PV device in which the light-absorbing layer is composed of copper, indium, gallium and selenium (a CIGS device), the CIGS layer (or thin film absorber layer) is the most difficult in the device stack to form and control. Similar issues over quality control exist for fabricating other thin film absorber layers, e.g., a thin film of cadmium telluride (CdTe) for a CdTe device. In the case of a CIGS device, control is difficult compared to other semiconductors because there are four constituent elements to control and fabrication may be complicated by sodium doping, high temperatures, and grading through the thin film. Unfortunately and undesirably, the quality of the CIGS layer is typically not known until after the entire device is manufactured with quality control tests performed electrically with contacts connected to upper and lower conductor layers. This situation precludes real time evaluation and optimization of the CIGS layer.
Cu(In, Ga)Se2 (CIGS) solar cells have achieved efficiencies in excess of 20 percent. CIGS devices are also able to be manufactured with various different manufacturing processes and techniques. Accordingly, CIGS is a leading candidate to displace silicon in the photovoltaics market. Nevertheless, there is still a challenge to characterize fundamental CIGS properties, such as carrier concentration and recombination in absorber layers. Further, without real time feedback on specific electro-optical properties, it is difficult to distinguish good from bad material in deposited thin films, to understand performance variations, to optimize growth processes, and to control this complex quaternary material. As noted above, it is desirable to provide similar real time quality control during the forming or deposition of other thin film absorber layers rather than having to wait to perform such testing on the finished PV device via electrical techniques.
The foregoing examples of the related art and limitations related therewith are intended to be illustrative and not exclusive. Other limitations of the related art will become apparent to those of skill in the art upon a reading of the specification and a study of the drawings.